Solar radiation in the spectral range 400 to 700nm, known as Photosynthetically Active Radiation (PAR), provides the energy required by terrestrial vegetation to produce organic materials from mineral components. The part of this PAR that is effectively absorbed by plants is called the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). It is a non-dimensional quantity varying from 0 (over deserts) to 1 (at least for large homogeneous canopy layers observed by medium- to low-resolution
sensors), although such high values are never witnessed in practice because some of the incoming light is always reflected back by the canopy or the underlying ground. FAPAR is related to, but different from, Leaf Area Index (LAI; covered later in the document) because canopy structure affects both ECVs.
FAPAR plays a critical role in assessing the primary productivity of canopies, the associated fixation of atmospheric carbon dioxide, and the energy balance of the surface. As is the case with land surface albedo (see this ECV earlier in this document), FAPAR depends on the illumination conditions, i.e., the angular position of the sun with respect to the vegetation layer and the relative contributions of the direct and diffuse irradiances. Both black-sky (assuming only direct radiation) and white-sky (assuming that all the incoming radiation is in the form of isotropic diffuse radiation) FAPAR values may be considered. Models describing the primary productivity of plants and the energy balance of the land surface require either a characterization of the diurnal evolution of FAPAR or the daily integrated value of FAPAR, depending on the time step used. Other applications may only require cumulative or aggregated values over longer periods.
For the purpose of environmental applications and carbon cycling, estimating the absorption of radiation by leaves is the primary objective, but other plant elements (trunks, branches, etc.) of the canopy also absorb or scatter radiation. The expression 'green FAPAR' is sometimes used to designate the value of FAPAR that is exclusively due to photosynthesizing materials (mostly leaves), i.e. not including scattering and absorption through other processes. FAPAR is difficult to measure directly in the field: In situ
estimates require the simultaneous measurement of all incoming and outgoing radiation fluxes into and out of the canopy layer, as well as the acquisition of architecture information to account for the absorption by canopy elements other than leaves (in particular for complex three-dimensional canopies such as forests). Specific problems (e.g. poorly designed measurement protocols) and ubiquitous deficiencies (e.g. horizontal fluxes of radiation that are rarely accounted for) frequently plague current experimental setups. They severely limit the feasibility of effectively comparing FAPAR values derived from space- based instruments with those derived from in situ measurements:
• While total PAR irradiance is typically monitored as part of the standard observation protocol at ecological and radiation research sites (e.g. FLUXNET, LTER, and SURFRAD), few of these sites generate all the necessary other measurements required to close the radiation budget and derive a reliable estimate of the canopy FAPAR at the scale of the observing space-borne sensor.
• A very detailed sampling strategy (e.g. at spatial intervals much smaller than the typical sampling distance of space-based sensors and consistent with the size of leaves and gaps in the plant canopy) is required in these field campaigns because FAPAR is highly variable in space and time. However, this is rarely implemented.
Information from PAR flux meters or directional PAR meters (e.g. the Ceptometer) inserted at the bottom of the canopy layer can be used to approximate the hemispherically integrated FAPAR (the latter by sampling over several directions in a short time period). Similarly, interception as derived from devices measuring the directional gap fraction (hemispherical photographs, LAI2000) can be used as proxies but with a lower accuracy. Significant improvements could be implemented in field measurements, especially in terms of measuring all relevant radiation fluxes and obtaining more representative spatial sampling statistics to account for the high variability of vegetation. FAPAR is also conditioned by the brightness of both the background and the canopy constituents, such that the accuracy of standard field measurements may decline under snowy conditions.
Global, gridded FAPAR products are routinely generated by Space Agencies and other institutional providers at a typical spatial resolution of 1km. Regional products may be available on finer scales of 250- 300m. These remote sensing products are derived by numerically inverting physically-based radiative transfer models against satellite measurements, typically reflectance observations from a wider spectral region than PAR because NIR and SWIR radiances are needed to account for the contribution of the background. By the same token, observations in the blue spectral band, near the edge of the PAR region, are important to help assess the influence of atmospheric aerosols on the measurements.
The obscuring of the surface by clouds introduces spatial discontinuities in the maps of FAPAR derived from single orbital overpasses. To improve the spatial coverage while maintaining the capability of documenting the phenology of vegetation, individual estimates are composited over standard periods, such as a week, ten days or a month.
The following is therefore required for this ECV:
Product T.7 Maps of the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) Benefits
• Provision of documentation of the primary productivity of ecosystems and characterization of the phenology of vegetation in space and time;
• Contribution to the estimation of atmospheric carbon dioxide sequestration in terrestrial environments, in combination with vegetation models and other sources of data.
Target Requirements Variable/ Parameter Horizontal Resolution Vertical
Resolution Temporal Resolution Accuracy Stability FAPAR 250m N/A 2-weekly averages (based on daily sampling) max(10%; 0.05) max(3%; 0.02)
Rationale: The horizontal resolution (250m) indicated here points to the typical values achievable today and matches land-cover-resolution targets. It is understood that carbon cycling models might be driven by spatially integrated values at a coarser resolution. The frequency of product delivery (no more than two- weekly) is driven by the need to detect changes in primary productivity of ecosystems (e.g. changes in length of growing season) and relies on a minimum observation time resolution of one day, taking into account the presence of clouds and other factors. The accuracy requirements noted in the preceding table are set to resolve significant regional changes and are near the limit of what can be achieved with mono- angular sensors (percentages indicate relative values). The temporal resolution could be significantly improved either by deploying a constellation of polar-orbiting sensors or with instruments on geostationary platforms, but the latter would be useful only if the spatial resolution would match, at least roughly, the indicated requirements (100m to 1km).
Requirements for satellite instruments and satellite datasets FCDR of VIS/NIR multispectral imager radiances, for example from:
• Instruments with a minimum number of three narrow spectral bands (blue, red and near-infrared), although some further improvements might be expected from a few additional bands or from the analysis of data generated by multi-directional sensors;
• Future space-based optical instruments, with spatial resolutions not inferior to current sensors (100m to 1km) and essentially global coverage in one or two days.
All current methods to retrieve FAPAR rely on the availability of co-located simultaneous measurements in multiple spectral bands and from multiple observation angles (when available). Image co-registration must be guaranteed to be no worse than one-third of the instantaneous field of view of a single detector, which translates into stringent requirements for orbital control, including pointing stability and knowledge.
To ensure the continuity of the FAPAR product across multiple sensors and platforms, a minimum overlap of one year (a full seasonal cycle) should be arranged between successive missions to allow for the inter- comparison and inter-calibration of sensors and the eventual adjustment of retrieval algorithms.
Calibration, validation and data archiving needs
• Accurate radiometric and spectral calibration mechanisms or procedures to ensure the stability of the measurements are required to guarantee the performance of the FAPAR products, especially in terms of detecting subtle trends and progressive changes that may result from expected climate changes;
• Despite the inherent difficulties of comparing space-based measurements with in situ observations, field measurements remain an essential component of the validation process. The availability of higher spatial resolution sensors (1 to 50m) with spectral and directional characteristics similar to the standard global sensors and the establishment of a reference network of experimental sites where FAPAR is measured in situ, at scales comparable with the spatial sampling frequency of satellite
observations, currently provide the basis for demonstrating the reliability and accuracy of the generated products; further efforts need to be made to develop and promote standard protocols to measure FAPAR in the field, and these must be coordinated with similar requirements arising from the albedo and LAI ECVs;
• Efforts must be undertaken to develop a traceable quality assurance system allowing the provision of unbiased and reliable evaluations of both in situ methodologies and EO retrieval algorithms for FAPAR, irrespective of differences in definitions, illumination conditions and spatial resolution.
Adequacy/inadequacy of current holdings
Space agencies and other institutional providers generate various FAPAR products at different temporal and spatial resolutions over the globe. Over ten years of space-derived FAPAR products are now available from different sources, at spatial resolutions typically in the range of 1 to 2km and temporal resolutions, such as daily, weekly, every ten days or monthly. Comparing these products reveals discrepancies that are mainly due to differences between concepts and definitions, retrieval methodologies or input-data quality. Periodic satellite data re-processing exercises, to take advantage of new findings and especially improvements in instrument calibration, have improved the reliability and consistency of these products. They should be repeated in the future. The following activities would significantly enhance the value and reliability of existing and forthcoming FAPAR products:
• Studies should be carried out to understand and reduce large systematic biases among the magnitudes of existing products, which otherwise exhibit generally consistent seasonal variability;
• Networks of in situ experimental sites should be expanded to become representative of a wider range of biomes and consolidated to be more consistent with the spatial scales of satellite observations; this effort should contribute to both an evaluation of field measurements and ultimately a more definitive validation of FAPAR products derived from space measurements;
• FAPAR products at spatial resolutions on the order of 100 to 300m are feasible – though not operationally generated today – from sensors such as MODIS, MISR, MERIS, and the like;
• For some applications, such as diagnostic or prognostic climate models, users may require separate FAPAR values for direct and diffuse incoming radiation, parameterized with respect to the solar zenith angle, although no institution is currently offering such products;
• The documentation associated with FAPAR datasets should include detailed information on the assumptions and models used in the generation of the product and, in particular, on the spectral properties, architectural structure, illumination, and observation angular conditions assumed to facilitate their interpretation in the context of field measurements and user applications.
Immediate action, partnerships and international coordination
• Space agencies and data providers should continue to generate FAPAR products but with greater emphasis on traceability, clarity of assumptions and documentation;
• Efforts should be directed toward clarifying the explicit or implicit definitions of FAPAR used in the generation of each product, on better defining the specific needs of various users, and on encouraging the convergence toward one or a very limited number of clearly documented products matching these requirements;
• Because of the presence of appreciable interannual variability, small trends in FAPAR can only be detected by analysing long time series; data providers should thus schedule the reprocessing of their archives to generate comparable and consistent products suitable for this purpose, including full documentation and traceability to international standards;
• Further efforts should also be made to reanalyse the historical archives of earlier instruments such as NOAA AVHRR (especially LAC observations, whenever available) and to extend FAPAR records into the past (going back to the mid-eighties), while ensuring compatibility and consistency with current records (see also C.7);
• Building on past and current expertise from existing programmes such as FLUXNET, SURFRAD and LTER, space agencies are encouraged to support a concerted effort to expand and strengthen a reference network of in situ field stations to monitor FAPAR (and associated ECVs such as albedo and LAI) that will design and implement observation protocols consistent with the spatial and temporal characteristics of space-based observations; these stations should also acquire ancillary information necessary for the proper interpretation and exploitation of these data for validation purposes;
• The research community can contribute to this effort and improve the FAPAR retrieval methodology by extensively testing the performance of algorithms in computer models of the environment, where all relevant conditions and properties can be controlled explicitly;
• A traceable quality assurance system should be developed, allowing the provision of unbiased and reliable assessments of the compliance of in situ and satellite-based FAPAR estimates with the GCOS criteria on accuracy (and precision), ideally taking into account differences in FAPAR definitions, illumination conditions and sensor footprints.
Link to GCOS Implementation Plan
• [IP-10 Action T31] Operationalize the generation of FAPAR and LAI products as gridded global
products at spatial resolution of 2km or better over as lengthy time periods as possible;
• [IP-10 Action T29] Establish a calibration/validation network of in situ reference sites for FAPAR and
LAI and conduct systematic, comprehensive evaluation campaigns to understand and resolve differences between the products and increase their accuracy;
• [IP-10 T3] Develop a subset of current LTER and FLUXNET sites into a global terrestrial reference
network for monitoring sites with a sustained funding perspective and with co-located measurements of meteorological ECVs; seek linkage with Actions T4 and T29, as appropriate.
Other applications
FAPAR products are useful in a number of applications, ranging from agriculture (e.g. crop-yield forecasting) and forestry to environmental stress and risk assessments (e.g. drought); they have been used to evaluate food-security issues, land degradation (e.g. desertification), and as one of the inputs for land-cover mapping.